The present invention relates to mine roof support systems and, more particularly, to a post-tensionable, resin anchored rod with a novel nut member having a portion which is breakable upon application of excess torque after the resin has set sufficiently to prevent continued rotation of the rod threadedly engaged with the nut member.
One of the most popular types of mine roof support is the resin-anchored, elongated rod (i.e., a bolt or rebar), wherein a two-compartment cartridge is fractured within a drill hole in the mine roof and the components mixed by advancement and rotation of the rod. A short time after mixing, the resin mix hardens to anchor the rod securely in the drill hole, thereby holding a bearing plate carried by the rod, in supporting engagement with the mine roof surrounding the drill hole entry in the roof.
It is also common in such roof support systems to provide means for tensioning the rod after the resin has set to produce compressive forces further reinforcing the rock strata of the mine roof. Beginning with the roof support system of U.S. Pat. No. 3,877,235, the elongated rods have been threadedly engaged with a nut, or other threaded member with a stop means to limit advance of the rod into or through the nut, rotationally coupling the rod to the nut. Thus, rotation of one of the elements is transmitted to the other to cause breakage of a two-compartment resin cartridge and mixing of the components within the mine roof drill hole. After sufficient setting of the resin mix, application of excess torque fractures or otherwise displaces the stop means, permitting further relative rotation of the rod and nut member, thereby drawing the support plate into close engagement with the roof about the drill hole and tensioning of the rod.
Among the resin-anchored, tensioned rod roof support systems wherein a relative rotation stop means is employed are those combining an expansion anchor with a resin anchorage, as exemplified in U.S. Pat. Nos. 4,160,614, 4,162,133 and 4,419,805. Other support systems of this general type, such as those shown in U.S. Pat. Nos. 4,051,683, 4,100,748 and 4,122,681 combine two elongated rods with an intermediate coupling member wherein the stop means is incorporated. Still others employ a nut member on a threaded end of the elongated rod which extends through the roof support plate with stop means rotationally coupling the nut, to which the rotative force is applied, to the rod until the latter is anchored by the resin and excess torque applied to the nut overcomes the stop means. Support systems of the latter type, with which the present invention is concerned, are disclosed in, among others, U.S. Pat. Nos. 3,979,918, 4,295,761 and 4,347,020.
In the support systems which include a nut on the lower, threaded end of the anchored rod, one or more additional elements are often employed to form the stop means. That is, a stop plug, disc, shear pin, or the like, is incorporated in or with the usual body portion having an internally threaded opening. Although drilling and tapping operations are required for the special roof support system nut members, as with any machined nut, the cost of both added parts and further manufacturing operations involved in incorporating the part(s) forming the stop means adds to the overall cost of the unit. It is the customary construction of such systems that the threaded, lower end of the rod extend through and below the nut member when the rod is tensioned, thus extending downwardly into the mine passage, an undesirable condition, particularly in mine passages of limited height, i.e., in "low coal" seams.
It is a principal object of the present invention to provide a mine roof support system wherein an elongated rod is anchored in a drill hole by a resin grouting at its upper end, and a nut element having a breakable stop means is installed on the lower, threaded end of the rod to effect tensioning thereof, wherein the system is particularly suited for use in low coal areas.
Another object is to provide a nut element for use in a mine roof support system and incorporating means breakable by application of excess torque to the nut when threadedly engaged with a resin-anchored rod, wherein the nut element is cast as an inegral unit and no operations are required to complete the nut other than drilling and tapping to provide the internal threads.
A further object is to provide a resin anchored, post tensionable, mine roof support system employing a malleable iron nut for threaded engagement with the lower end of a tensioned rod wherein the nut is rotated by a male wrench engaged with an internal socket in the base of the nut and the lower end of the rod does not extend beyond the lower end of the nut.
Other objects will in part be obvious and will in part appear hereinafter.